Flow cytometry technology is a powerful tool to study cancer cell biology. Using monoclonal antibodies, molecular probes and chemicals labelled with flurochromes with different excitation- emission spectra single and multiple cell surface markers, apoptosis, cell activation status, cell cycle kinetics, gene and gene product expression can be measured in individual cells. Moreover, this technology has the capacity to rapidly isolate cells that display common or different parameters within a mixed population. The Aims of this proposal are to provide: 1) State-of-the-art instrumentation, b) professional oversite, and c) technical support for NCI supported investigators conducting a variety of studies in cancer cell biology. These funded studies include the following: Multi-drug resistance in mammalian cells and mechanism of drug resistance in acute myeloid leukemia, folate transport and antifolate resistance, regulation of the DNA synthesone in normal and malignant breast cells, characterization of the cytotoxic drug resistance phenotype of cells expressing the breast cancer resistance protein, mechanism of IL-10 activity in breast cancer therapy, investigation of ErbB receptor control of breast cancer cell growth, the role of IGFR (I and II) in GI malignancies, and the role of microsatellite instability in colorectal cancers. Flow cytometry technology will greatly enhance the ability of these investigators to achieve the goals and aims of their research efforts.